Open AccessReview Intrinsic and extrinsic factors influencing the clinical course of B-cell chronic lymphocytic leukemia: prognostic markers with pathogenetic relevance Michele Dal-Bo1,
Trang 1Open Access
Review
Intrinsic and extrinsic factors influencing the clinical course of B-cell chronic lymphocytic leukemia: prognostic markers with
pathogenetic relevance
Michele Dal-Bo1, Francesco Bertoni2, Francesco Forconi3,
Antonella Zucchetto1, Riccardo Bomben1, Roberto Marasca4, Silvia Deaglio5, Luca Laurenti6, Dimitar G Efremov7, Gianluca Gaidano8, Giovanni Del
Poeta9 and Valter Gattei*1
Address: 1 Clinical and Experimental Onco-Hematology Unit, Centro di Riferimento Oncologico, I.R.C.C.S., Aviano (PN), Italy, 2 Laboratory of Experimental Oncology and Lymphoma Unit, Oncology Institute of Southern Switzerland, Bellinzona, Switzerland, 3 Division of Hematology and Transplant, Department of Clinical Medicine and Immunological Sciences, University of Siena, Siena, Italy, 4 Division of Hematology –
Department of Oncology and Hematology-University of Modena and Reggio Emilia, Modena, Italy, 5 Laboratory of Immunogenetics, Department
of Genetics, Biology and Biochemistry and CeRMS, University of Turin, Turin, Italy, 6 Hematology Institute, Catholic University "Sacro Cuore", Rome, Italy, 7 Molecular Hematology, ICGEB Outstation-Monterotondo, Rome, Italy, 8 Division of Hematology – Department of Clinical and
Experimental Medicine & BRMA – Amedeo Avogadro University of Eastern Piedmont, Novara, Italy and 9 Chair of Hematology, S.Eugenio Hospital and University of Tor Vergata, Rome, Italy
Email: Michele Dal-Bo - micheledalbo@gmail.com; Francesco Bertoni - frbertoni@mac.com; Francesco Forconi - forconif@unisi.it;
Antonella Zucchetto - antonellazucchetto@libero.it; Riccardo Bomben - riccardo.bomben@gmail.com; Roberto Marasca - marasca@unimo.it; Silvia Deaglio - silvia.deaglio@unito.it; Luca Laurenti - l.laurenti@rm.unicatt.it; Dimitar G Efremov - efremov@icgeb.org;
Gianluca Gaidano - gaidano@med.unipmn.it; Giovanni Del Poeta - g.delpoeta@tin.it; Valter Gattei* - vgattei@cro.it
* Corresponding author
Abstract
B-cell chronic lymphocytic leukemia (CLL), the most frequent leukemia in the Western world, is
characterized by extremely variable clinical courses with survivals ranging from 1 to more than 15
years The pathogenetic factors playing a key role in defining the biological features of CLL cells,
hence eventually influencing the clinical aggressiveness of the disease, are here divided into
"intrinsic factors", mainly genomic alterations of CLL cells, and "extrinsic factors", responsible for
direct microenvironmental interactions of CLL cells; the latter group includes interactions of CLL
cells occurring via the surface B cell receptor (BCR) and dependent to specific molecular features
of the BCR itself and/or to the presence of the BCR-associated molecule ZAP-70, or via other
non-BCR-dependent interactions, e.g specific receptor/ligand interactions, such as CD38/CD31 or
CD49d/VCAM-1 A putative final model, discussing the pathogenesis and the clinicobiological
features of CLL in relationship of these factors, is also provided
Introduction
B-cell chronic lymphocytic leukemia (CLL) is a
mono-clonal expansion of small mature B lymphocytes
accumu-lating in blood, marrow, and lymphoid organs Despite a remarkable phenotypic homogeneity, CLL is character-ized by extremely variable clinical courses with survivals
Published: 28 August 2009
Journal of Translational Medicine 2009, 7:76 doi:10.1186/1479-5876-7-76
Received: 27 June 2009 Accepted: 28 August 2009
This article is available from: http://www.translational-medicine.com/content/7/1/76
© 2009 Dal-Bo et al; licensee BioMed Central Ltd
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Trang 2ranging from one to more than 15 years [1] In this regard,
specific chromosomal aberrations (i.e 17p-, 11q- or +12),
as well as the presence of an unmutated (UM) rather than
mutated (M) status of immunoglobulin (IG) heavy chain
variable (IGHV) genes, or expression levels for ZAP-70,
CD38 and CD49d exceeding the value of an established
threshold, have been reported to correlate with a poor
clinical outcome in CLL [2-8]
In the present review, the main factors playing a role in
defining the biological features of CLL cells, hence
eventu-ally influencing the clinical aggressiveness of the disease,
are divided into "intrinsic factors", mainly genomic
alter-ations of CLL cells, and "extrinsic factors", responsible for
direct micro-environmental interactions of CLL cells
Intrinsic factors
Under the terms "intrinsic factors" are gathered the major
genomic alterations associated with a CLL phenotype
Such alterations can be either primarily responsible for
the first step(s) of neoplastic transformation of B cells
(primary genetic lesions, e.g 13q14.3 deletion, see
below) or acquired during disease progression, also as a
consequence of microenvironmental interactions (i.e
sec-ondary genetic lesions) Telomer lenght too was included
in this chapter, although often consequence of
environ-mental factors affecting cell proliferation (see below)
It is common notion that, differently from other B-cell
lymphoid neoplasms, CLL is characterized by recurrent
DNA gains and losses and not by the presence of specific
chromosomal translocations However, using either
improved protocols to obtain informative metaphases
[9,10] or microarray-based comparative genomic
hybridi-zation [11], chromosomal abnormalities can now be
detected in over 90% of patients [9] Only a fraction of the
events are balanced translocations, whilst the vast
major-ity of them are unbalanced translocations (see below),
determining losses or gains of genomic material [9,10]
Specific genomic events are associated with a different
clinical outcome and, the frequency of specific genomic
events varies between CLL bearing Mutated (M) and
Unmutated (UM) IGHV genes (see below for IGHV
molecular features) The recurrent chromosomal aberra-tions are summarized in Table 1
13q14.3 deletion
The most common lesion in CLL is chromosome 13q14.3 deletion, occurring in half of the cases [4] The deletion is often interstitial and can be homozygous in up to 15% of the cases [4] When it represents the only lesion it is asso-ciated with a good clinical outcome, and with the
pres-ence of Mutated IGHV genes [4,10,12] A selective
advantage, possibly proning B cell clones to additional mutations, could be conferred because of the high fre-quency of 13q deletion [13]
The pathogenetic role of 13q deletion in CLL is not fully clear, although its high frequency has suggested a primary and central role in the CLL transformation process [14] Several regions between 130 and 550 kb were described, all comprising a minimal deleted region of 29 kb located
between exons 2 and 5 of DLEU2 [15] The deleted region
always comprises the locus coding for two microRNAs (miRNAs), hsa-mir-16-1 and hsa-mir-15a [15], but it can also include the region coding for the retinoblastoma
gene (RB1) [16] mir-16-1 and mir-15a are deleted or
downregulated in the majority (about 70%) of CLL [14] miRNAs represent a large class of regulating non-coding small RNA molecules, acting by binding messenger RNAs and determining their degradation or inhibition of
trans-lation [17] Over-expression of the anti-apoptotic BCL2, due to the reduced negative regulation by mir-16-1 and
mir-15a, has been proposed along with other several genes
often involved in cell cycle and/or programmed cell death
regulation such as MCL1, ETS1 and JUN [16,18-20]
Addi-tional studies are needed to identify the genes actually involved in CLL pathogenesis via the 13q deletion
Trisomy 12
The trisomy 12 bears an intermediate prognosis and is
only marginally associated with an UM IGHV gene status [10,12] The 12q22 segment contains CLLU1 which is the
first gene that was considered specific for CLL cells, but no
Table 1: Intrinsic factors with prognostic relevance
Karyotype % of cases, range a Prognosis Known and/or putative involved genes
a According to [30];
b If the sole genetic aberration.
Trang 3difference in CLLU1 protein expression in patients with or
without trisomy 12 has been reported [21,22] Of note,
high CLLU1 expression levels has been demonstrated to
predict poor clinical outcome in CLL of younger patients
[23]
11q22-q23 deletion
CLL harboring 11q22-q23 deletion tend to present a
rap-idly evolving disease [4] This lesion targets the gene
cod-ing for ATM (ataxia telangiectasia mutated), which is
mutated in approximately 15% of CLL, not necessarily
bearing concomitant 11q losses [24] The presence of 11q
deletion or of ATM mutations determines poor prognosis,
and it is more common among cases with UM IGHV and
ZAP-70 or CD38 positivity, or experiencing bulky
lym-phadenopathies [4,10,24-28] ATM is involved in the
DNA repair and its inactivation impairs the response of
CLL cells to chemotherapy [26,28] It has been suggested
that, for the complete lack of ATM function, the other
ATM allele should present mutations [29] Since ATM
mutations are present in one third of the 11q- cases, the
poor prognosis of 11q- patients has been suggested to
depend on mechanisms involving other genes affecting
cell cycle regulation and apoptosis (e.g NPAT, CUL5,
PPP2R1B) [28,29].
17p13.1 deletion
The recurrent 17p13.1 deletion, affecting TP53, occurs
only in a small fraction of CLL patients at diagnosis [4] It
confers the worst prognosis among all the genetic lesions
[4], and it is more common among patients bearing other
poor prognostic factors, such as UM IGHV, or ZAP-70 and
CD38 expression [4,10,27,30] TP53 is a transcription
fac-tor activated by strand breaks in DNA that is involved in
triggering cell apoptosis and/or cell-cycle arrest, with the
aim to maintain the genome integrity by hindering clonal
progression [31] The activation of TP53 is tightly
regu-lated by the MDM2 (murine double minute-2) gene [32],
whose expression is regulated in part by a TP53 responsive
promoter MDM2, an E3 ubiquitin ligase for TP53 and
itself, controls TP53 half-life via ubiquitin-dependent
deg-radation [33-35] In cells with functional TP53, the TP53
activity is primarily inhibited through direct and tonic
interaction with the MDM2 protein [32] Treatment of
various tumor cells with inhibitors of the MDM2-TP53
interaction results in rising TP53 levels and subsequent
induction of cell cycle arrest and apoptosis [36] Thus,
small-molecule inhibitors that block the MDM2-TP53
interaction, like Nutlins, could represent a new
therapeu-tic strategy for treatment of CLL patients [37]
In CLL, TP53 is mutated in about 10% of patients at
pres-entation and in 10% to 30% of patients with pretreated
disease [38-40] TP53 can be inactivated by somatic
muta-tions which can occur in the presence or in the absence of
any genomic loss [2,25] Whereas up to two-thirds of
del17p13 CLL also harbor TP53 mutations, a fraction of CLL carries TP53 mutations without del17p13 [2,25,41], and TP53 mutations have been shown to have a negative prognostic relevance also in the absence of TP53 deletion [42] Besides TP53 mutations and deletion, other
mecha-nisms of TP53 dysfunction may be operative in CLL
[28,43-46] These mechanisms may involve the ATM and
MDM2 genes that regulate TP53 function at the protein
level [28,46] ATM is related to TP53 because it acts as a
TP53 kinase, although ATM deletions do not confer a dis-ease as aggressive as it occurs in TP53 deletions [47] Nota-bly, ATM mutations and MDM2 polymorphisms causing
aberrant MDM2 expression have been shown to harbor prognostic relevance in CLL [28,43,46]
TP53 inactivation is associated with a poor response to chemotherapy, including alkylating agents and purine analogues [2] This suggested the need, for patients affected by CLL with disrupted TP53 function, of TP53 independent therapeutic agents [26,41,48,49] In this regard, CLL that at diagnosis presented del17p13 without
TP53 mutations displayed a significantly longer time to
chemorefractoriness than CLL with TP53 mutations
already at diagnosis [42] In addition, CLL with del17p13
only acquired TP53 mutations at chemorefractoriness
[42]
Chromosomal translocations and other chromosomal abnormalities
Historically, chromosomal translocations were consid-ered infrequent events in CLL However, relatively recent studies reported an unexpected high frequency (approxi-mately 20%) of reciprocal translocations when successful methods for CLL B cell stimulation are employed, e.g by utilizing CD40 ligand or oligonucleotides and IL-2 as stimuli [9,50] These studies have also correlated chromo-somal translocations with shorter treatment-free survival and overall survival Together with the more common chromosomal abnormalities, genome wide screening has found other alterations consisting of clonal monoallelic and biallelic losses as well as gains such as duplications, amplifications and trisomies [51-54] These alterations concern relatively small chromosomal regions spread throughout the CLL genome [51-54] Moreover, these gains or losses enable the detection of clonal variants that differ at several loci [52] The biologic and prognostic sig-nificance of these other recurrent genomic aberrations is not known Patients bearing three or more aberrations or chromosomal translocations might have a worse progno-sis [9] Prospective trials and a more widespread use of genome wide techniques to assess CLL genome will help
to identify further genetic prognostic markers
Telomere length
An interesting feature of CLL is its heterogeneity in terms
of telomere length and telomerase (hTERT) activity [55-58] Short telomeres and high hTERT activity are
Trang 4associ-ated with worse clinical outcome, with an UM IGHV gene
status, with high ZAP-70, CD38, and CD49d expression,
as well as with specific cytogenetic abnormalities
[56,58,59] Regarding this latter point, short telomeres are
frequently associated with 11q or 17p deletions whereas
long telomeres are present in 13q- patients [58] Normal
B cells in the germinal center present high hTERT activity,
and telomere elongation has been shown to occur at the
same time of the somatic hypermutation process [60],
thus, B cells with M IGHV genes present longer telomeres
than B cells with UM genes Therefore it is conceivable
that different B cells already present different telomere
length before the leukemic transformation; alternatively,
kinetic characteristics of CLL cells can determine
differ-ences in telomere length, and telomere shortening might
be a consequence of 11q- or 17p- aberration that, together
with ZAP-70, CD38 and CD49d overexpression, results in
a more rapid CLL cell turnover, facilitating survival and
cell-cycle progression [58,61]
Clinical implications of intrinsic factors
In the clinical practice, the detection, by using a panel of
interphase fluorescence in situ hybridization (FISH)
probes, at least including 13q14.3, 11q22-23 and
17p13.1 deletions and trisomy 12, should always be part
of the initial diagnostic procedure Although only a small
portion of patients presents genetic abnormalities
consid-ered bad prognostic markers, such as 17p or 11q
dele-tions, at the onset, these alterations can appear during the
clinical course, more often in patients carrying other poor
prognostic markers (such as UM IGHV mutational status
or high ZAP-70, CD38 and CD49d expression) [38,39]
Given that acquisition of new cytogenetic abnormalities
may influence the response to therapy, FISH analysis
should be repeated at the time of progression or before
therapy selection Given its valuable prognostic impact,
analysis of TP53 mutational status could be also advisable
in the phase of progressive disease
Extrinsic factors
Extrinsic factors are responsible for direct interaction of CLL cells with other micro-environmental cell popula-tions In the present review, we focused on interactions of CLL cells occurring via the surface B cell receptor (BCR) and dependent on specific molecular features of the BCR itself and/or on the presence of the BCR-associated mole-cule ZAP-70, or via other non-BCR-dependent interac-tions, e.g the CD38/CD31 or CD49d/VCAM-1 receptor/
ligand interactions (Table 2) Differences in IGHV
muta-tional status and in BCR funcmuta-tionality suggested a differ-ent cell of origin for CLL with UM versus CLL with M
IGHV gene mutational status Despite this, CLL cases
appear very homogenous when their gene expression pro-files are compared with those of normal or other neoplas-tic B-cells [62,63] For this reason CLL is nowadays believed to derive from subsets of marginal zone memory B-cells that have undergone either a cell dependent or T-cell independent maturation [64,65]
The BCR in CLL
BCR is a multimeric complex constituted of a membrane-bound IG glycoprotein and a heterodimer IGα/IGβ (CD79A/CD79B), located on the surface of B cell The IG glycoprotein is composed by two identical heavy chains (μ, δ, α, γ or ε) and two identical light chains: κ or λ Both
heavy and light chains have two variable regions (IGHV or
IG(K/L)V) that mediates antigen contact and vary
exten-sively between IG, along with a constant region that is responsible for the effector activities For heavy chain, the variable region is encoded by three gene segments:
varia-ble (IGHV), diversity (IGHD) and joining (IGHJ), whereas
the variable regions of the light chains are generated from
Table 2: Extrinsic factors with prognostic relevance
Factors Negative prognosis if expressing Cases with unfavourable values, mean
% (range)
Putative mechanisms responsible for unfavourable prognosis
BCR - UM IGHV
- stereotyped BCR?
- M IGHV3-23?
42.3 (40–46) a - high reactivity or polyreactivity
- superantigens recognition?
- calcium influx
- chemokine sensitivity
(CD49d/VCAM-1; CD49d/Fibronectin)
a Deduced from [25,91,92];
b Deduced from [7,114,116];
c Deduced from [25,127,128];
d Deduced from [8,150,152].
Trang 5IG(K/L)V and IG(K/L)J segments Both for heavy and light
chains, the segments involved in V(D)J recombination
confer diversity by random and imprecise rearrangement
during B-cell development in the bone marrow The
con-sequent protein sequences mainly differ in the
comple-mentary-determining-region-3 of the heavy (HCDR3)
and light (K/LCDR3) chains Diversity is further enhanced
by the somatic hypermutation (SHM) process, which
requires BCR cross-linking by the antigen, cellular
activa-tion, cooperation of T lymphocytes and other cells, and
introduces point mutations in variable regions of
rear-ranged immunoglobulin heavy and light chains [66]
Another process physiologically occurring during B cell
differentiation is the so-called class-switch recombination
(CSR), which modify the constant region of heavy chains,
thus altering the effector functions of IG [66]
The BCR has always been a key molecule to understanding
CLL, initially only due to the surface IG that were utilized
to make or support a correct diagnosis [67] Surface IG are
usually IGM/IGD, expressed at low/dim intensity [47]
The explanation of the low/dim expression level of BCR is
still unclear [47] CLL expressing IGG is a relatively rare
variant whose origin and antigenic relation with the most
common IGM/IGD variant is still not completely clear
[68]
Studies of the molecular structure of the BCR in CLL are
suggesting evidences of a promoting role of the antigen
encounter A first evidence has been provided by analysis
of IGHV genes starting in the early 90s' that revealed that
50% of CLL had M IGHV genes [69-71] These mutations
often fulfill the criteria for selection by antigen with more
replacement mutations in heavy chain complementarity
determining regions (HCDR) and less in heavy chain
framework regions (HFR), which permits the
develop-ment of a more specific antigen-binding site by
maintain-ing the necessary supportmaintain-ing scaffold of BCR [6,72-76]
From a clinical point of view, in 1999, two mutually
con-firmatory papers demonstrated that somatic mutations
correlated with more benign diseases In fact, a CLL
sub-group with very unfavourable clinical outcome presents
none or few (<2%) mutations (UM CLL) in IGHV genes,
respect to the closest germ line sequence CLL cells of this
particular subgroup seem to receive continuous
anti-apoptotic and/or proliferating microenvironmental
stim-uli via BCR leading to a more aggressive disease than the
subgroup with M configuration of IGHV genes (≥2%; M
CLL), respect to the closest germ line sequence [3,77] A
difference in outcome was also demonstrated in patients
receiving an autologous stem-cell transplant (ASCT); all
patients with UM IGHV genes undergoing ASCT relapsed
and progressed after a 4-year follow-up, while most with
M IGHV genes remained in molecular remission at this
stage [78]
Activation-induced cytidine deaminase (AID), an enzyme involved in SHM and CSR during normal B cell differenti-ation [79], was found to be upregulated in UM CLL cells [80], and, even if expression could be restricted to a small fraction of the clone [6,81], AID seems to be functional with generation of isotype-switched transcripts and muta-tions in the pre-switch μ region [82,83] AID upregulation causes mutation in genes related with an aggressive
dis-ease (e.g BCL6, PAX5, MYC, RHOH) [84,85]
Further-more, a relation of AID expression with deletions in 11q-and loss of TP53 has been found [86]
BCR stereotypes in CLL (see Figure 1)
CLL have a biased use of some specific gene segments For
example, a preferential use of IGHV1-69 in the UM CLL and IGHV4-34 and IGHV3-23 in the M CLL subgroups has
been documented [87-92] In addition, several groups reported the existence of subsets of CLL cases carrying BCR characterized by non-random pairing of specific
IGHV, highly homologous or identical HCDR3 often
associated with a restricted selection of IGVK or IGVL light
chains (the so-called "stereotyped BCR") [89-97] These stereotyped BCR have been detected in more than 20% of CLL cases [89,91,92,97]; the non-random composition of the expressed BCR on the CLL cells with IG binding lead
to hypothesize a specificity for similar/identical antigens [89,91,92,98]
The chance of carrying a stereotyped BCR is higher for UM CLL [94] The vast majority of the clusters shared by dis-tinct, and in several cases geographically distant, datasets ("common" clusters) were composed by UM cases
[89,91-CLL subsets with distinct BCR features and their correlation with prognosis
Figure 1 CLL subsets with distinct BCR features and their cor-relation with prognosis Question marks in parenthesis
indicate data that has to be confirmed by further investiga-tions
Trang 694,97] In particular, these UM clusters included cases that
seem to express both autoreactive and polyreactive BCR,
allegedly deriving from the B cell compartment devoted to
the production of natural antibodies [96,98,99] Among
"common" clusters, of particular clinical interest is a
clus-ter composed by UM CLL with sclus-tereotyped BCR
express-ing genes from the IGHV1 gene family other than
IGHV1-69 (IGHV1-2,IGHV1-18, IGHV1-3,IGHV1-46,
IGHV7-4-1), homologous HCDR3 bearing the QWL amino acid
motif, and IGKV1-39 light chains [89,91,92] The
progno-sis of CLL expressing this stereotyped BCR is poor either if
compared to all the other patients affected by M or UM
CLL, or only to the cases expressing the same IGHV genes
but without the same stereotyped BCR [89,92]
Among the few M clusters that are shared by the majority/
totality of the datasets, there are two clusters, both
expressing IGG, composed by cases expressing IGHV4-34
and IGHV4-39, respectively [89,91,92,100,101] Specific
cluster-biased genomic aberrations have been found;
13q-has been associated with IGHV4-34/IGKV2-30 cluster
while trisomy 12 has been associated with the IGHV4-39/
IGKV1-39 cluster [101] Interestingly, the latter cluster has
been associated with the development of Richter
syn-drome in CLL [102,103] Other clusters, mainly
com-posed by M cases and expressing IGHV3 subgroup genes,
are less frequent and might be subjected to a geographical
bias
Finally, of particular interest is a group of IGHV3-21 CLL,
composed by cases with either UM or M IGHV genes, that
expressed a stereotyped BCR characterized by an
unusu-ally short and highly homologous HCDR3 associated
with IGLV3-21 [88,90,93,104,105] Of note, a
signifi-cantly skewed representation of this particular cluster has
been well documented in different European and
non-European countries and even in different regions from the
same country [88,90,104,105] From a clinical
stand-point, evidence is provided that patients belonging to
IGHV3-21/IGLV3-21 CLL cluster have shorter TTT when
compared to all M CLL and to M CLL expressing
IGHV3-21 but not included in this stereotyped cluster [88,90,92].
Although the issue is still controversial [88,105], the
molecular basis for a more aggressive clinical behaviour of
CLL belonging to IGHV3-21/IGLV3-21 CLL cluster is also
suggested by gene expression profiling and
immunophe-notypic analyses [90] The notion that only patients
affected by CLL belonged to the IGHV3-21/IGLV3-21
clus-ter experience a more progressive disease may have
impor-tant implications given the proposal of using IGHV3-21
expression to drive clinical decision in prospective trials
[27,49]
Non-stereotyped BCR in CLL (see Figure 1)
Considering the IGHV gene usage and relating it with the
distribution of IGHV gene in stereotyped BCR clusters, it
has been observed that cases expressing the IGHV3-23
gene are constantly absent from stereotyped BCR clusters
[106], despite that IGHV3-23 is the second most fre-quently used and usually M IGHV gene in CLL [89,90,92].
A possible explanation justifying the absence of IGHV3-23
genes from clusters of stereotyped BCR is the possibility
that IGHV3-23-expressing BCR might be selected through
non-CDR-based recognition mechanisms, e.g through interactions with superantigens, a general feature of BCR
expressing IGHV3 subgroup genes [106-109] From a
clin-ical standpoint, hints suggesting a negative prognostic
impact of IGHV3-23 usage in CLL have been reported
[110] Recently, such a suggestion has been confirmed in
an Italian multicenter series, but circumscribed to cases
expressing mutated IGHV genes [106] In this series, median TTT of M IGHV3-23 patients were significantly shorter than median TTT of M non-IGHV3-23 CLL, and
IGHV3-23 expression was identified as an independent
negative prognosticator in the context of M CLL [106]
ZAP-70
ZAP-70 encodes for T cell specific zeta-associated
protein-70 and has been initially identified in T cells as a protein tyrosine kinase that plays a critical role in T-cell-receptor signaling [111] This molecule is a member of the syk fam-ily of tyrosine kinases and is associated with the ζ-chain of the CD3 complex [112]
Gene expression profiling studies in CLL, aimed at identi-fying differentially expressed genes between UM and M CLL, described ZAP-70 as the most differentially expressed gene between the two CLL subtypes, thus high-lighting a high correlation between ZAP-70 expression and IGHV mutational status [63,113] Consistently,
ZAP-70 was shown to act as surrogate for IGHV gene mutations
when its intra-cytoplasmic expression is investigated by flow cytometry [5,7,114-116], although a common stand-ardized protocol for its detection is still to be defined [7,114,115,117] However, discordance of ZAP-70
expres-sion and IGHV mutational status was reported in about
25% of cases with a higher number of discordant cases in subgroups with a more aggressive disease such as
11q-CLL, 17p- CLL or IGHV3-21 CLL (39%) [118] Using a
cut-off set at 20% of positive cells, ZAP-70 expression was demonstrated to have a negative prognostic impact in CLL [5,7] The relevance of ZAP-70 as independent prognosti-cator was provided by multivariate analysis [116] ZAP-70 can modulate BCR-derived signaling associating with BCR in antigen stimulated CLL cells [119], and can play an indirect role in BCR signal transduction, mainly modulating events at the end of the signaling response [120] Expression of ZAP-70, which can enhance and pro-long on syk and other downstream signaling molecules, can partially determine the different capability of CLL cells to respond to antigenic stimulation [120] Regarding
Trang 7the mechanism(s) underlying the negative prognostic
impact of 70 expression in CLL, it is known that
ZAP-70+ CLL cells have a greater capacity to respond to
antigen-induced signals through BCR triggering In particular,
ZAP-70 expression and sustained BCR stimuli have been
associated with prolonged activation of the Akt and ERK
kinases, events which are required for the induction of
several antiapoptotic proteins, including Mcl-1, Bcl-xL
and XIAP [120-122] Recently, ZAP-70 expression was
demonstrated to mark CLL subsets with enhance
capabil-ity to respond to chemokine-mediated stimuli (see
below)
CD38
CD38 is a 45-kDa type II membrane glycoprotein first
described as an activation antigen whose expression
coin-cided with discrete stages of human T and B lymphocyte
differentiation [123] CD38 has been found to be widely
expressed in humans within the hematopoietic system
(e.g bone marrow progenitor cells, monocytes, platelets
and erytrocytes) and beyond, in brain, prostate, kidney,
gut, heart and skeletal muscle [124] CD38 behaves
simul-taneously as a cell surface enzyme and as a receptor As an
ectoenzyme, CD38 synthesizes cyclic adenosine
diphos-phate (ADP) ribose and nicotinic acid adenine
dinucle-otide phosphate (NAADP), key compounds in the
regulation of cytoplasmic Ca++ levels [125] Engagement
of CD38 by its ligand CD31 or by specific agonist
anti-bodies induces activation and differentiation signals in T,
B and NK cells [126] Signals mediated by CD38 are
tightly regulated by the dynamic localization of the
mole-cule in lipid microdomains within the plasma membrane,
and by lateral associations with other proteins or protein
complexes [124]
A study by Damle et al indicated that CD38 expression
was heterogeneous among CLL cases [3] By using a given
percentage of CLL cells expressing the antigen (30% of
positive cells), significant prognostic differences were
found by investigating both chemotherapy requirements
and overall survival [3] The same report showed that CLL
cells with higher CD38 expression more likely rearranged
UM IGHV genes [3] Thus, CD38 status was proposed as
surrogate for IGHV mutation status, although this was not
confirmed by subsequent studies, which however
sub-stantiated the its independent prognostic significance
[12,127-131]
These observations on the prognostic relevance of CD38
found a biologic ground in studies indicating that CLL cell
growth and survival were favoured through sequential
interactions between CD38 and CD31 and between
CD100 and plexin B1, the latter expressed by
microenvi-ronmental cells [132,133] These interactions are more
likely to occur in peripheral lymphoid organs and/or bone marrow given the higher CD38 expression in resi-dential as opposed to circulating CLL cells [134-136] Moreover, both bone marrow and peripheral lymphoid organs can provide accessibility to CD31, as endothelial, stromal, and the so-called nurse-like cells all express high-CD31 levels [137-139] Necessary condition for CD38-mediated signals are CD38 translocation into lipid rafts and lateral association with CD19, which is also part of the so-called "tetraspan web" (CD19/CD81), and com-prises different molecules, including β1 integrins such as CD49d [140] Moreover, CD38+ CLL cells, expecially if coexpressing ZAP-70, are characterized by enhanced migration toward CXCL21/SDF-1α, and CD38 ligation leads to phosphorylation of the activatory tyrosines in ZAP-70 [133,141] Therefore, ZAP-70 represents a cross-point molecule where migratory signals mediated via the CXCL21 receptor CXCR4 intersect with growth signals mediated via CD38 [142-144] Finally, the associated expression of CD38 and CD49d (see below) can provide additional mechanisms explaining the poor prognosis of CD38-expressing CLL
CD49d
CD49d, a.k.a α4 integrin, acts primarily as an adhesion molecule capable of mediating both cell-to-cell interac-tions, via binding to vascular-cell adhesion molecule-1 (VCAM-1), and interactions with extracellular matrix components by binding to non-RGD sites (a.k.a CS-1 fragments) of fibronectin (FN), as well as the C1q-like domain of elastin microfibril interfacer-1 (Emilin-1) [145,146] In this regard, CD49d-expressing CLL cells were shown to have a high propensity to adhere to fibronectin substrates, and an increased CD49d protein expression was demonstrated in CLL cells from advanced Rai stage patients [147] Our group recently collected evi-dences of VCAM-1 over-expression in the stromal-endothelial component found in the context of lymphoid aggregates in bone marrow biopsies (BMB) of CD49d/ CD38-expressing CLL [148] VCAM-1 upregulation was demonstrated to be due to an overproduction by CD38/ CD49d-expressing CLL cells of specific chemokines (CCL3 and CCL4) upon CD38 triggering, eventually capa-ble to recruit TNFα-producing macrophages, which in turn are responsible for VCAM-1 upregulation by stromal/ endothelial cells [148] VCAM-1/CD49d interactions resulted in an increased survival of CD49d-expressing CLL cells [148] CD49d-dependent interactions have a role in preventing both spontaneous and drug induced apoptosis
of normal or neoplastic B cells [145,149] Moreover, chemokine-induced transmigration of CLL cells across endothelia depends on CD49d expression by CLL cells and is favoured by the production of the matrix metallo-proteinase-9 as the result of CD49d engagement [150]
Trang 8From a clinical point of view, CD49d has been identified
as an independent negative prognosticator for CLL,
mark-ing a subset of CLL patients characterized by aggressive
and accelerated clinical course [8,150-152] The
prognos-tic relevance of CD49d in CLL may have also therapeuprognos-tic
implications, envisioning the use for CLL patients of
Natalizumab (TYSABRI, Biogen Idec, Cambridge, MA and
Elan Pharmaceuticals, South San Francisco, CA, USA), a
humanized anti-CD49d monoclonal antibody already
available and currently employed in autoimmune
dis-eases such as multiple sclerosis and Crohn's disease [153]
Conclusion (see Figure 2)
B cells carrying BCR with high affinity for autoantigens are
usually deleted or addressed towards a secondary
rear-rangement of heavy/light chains; in the latter case, B cells
that reach an "acceptable" ("non-autoreactive") structure are then driven to continue differentiation [154,155] In some istances, such secondary attempts may fail and B cell clones may retain an "inappropriate" reactivity (autoreac-tivity, polyreactivity) [156] As an example, many normal
B cell clones with UM IGHV genes produce antibodies
capable of a certain degree of polyreactivity by binding multiple antigens (e.g carbohydrates, nucleic acids, phos-pholypids) If one of these cells presents or develops pri-mary genetic abnormalities (e.g 13q14.3 deletions, but also other lesions) it can undergo leukemic transforma-tion B cells with genetic abnormalities and UM/polyreac-tive BCR can increase their number through repeated expositions to antigens (foreign antigens, autoantigens) [71,157] In this regard, immune cross-reactivity between exogenous polysaccharide/carbohydrate antigens and
A "multistep" model for CLL origin
Figure 2
A "multistep" model for CLL origin.
Good prognosis
Bad prognosis
CLL with
“Unmutated” BCR
Acquisition of additional
genetic abnormalities
Aggressive
phenotype
Extr insic Factor s
Ŷ%&5 DXWRUHDFWLYLW\
and/or polyreactivity
Ŷ=$3 KLJK
Ŷ&' KLJK
Ŷ&' G KLJK
CLL with
“Mutated” BCR
With SHM
Intr insic factor s Genetic damage
Extr insic factor s Ŷ%&5³0RQR´UHDFWLYLW\ Ŷ=$3 ORZ
Ŷ&' ORZ Ŷ&' G ORZ
High proliferation rate
Without SHM
Low proliferation rate
Increased cell trafficking
Low apoptotic rate
Low apoptotic rate
B cell (“auto/polyreactive”??)
Trang 9autoantigens is not infrequent [158,159] Together with
BCR, other factors, usually highly expressed in UM CLL,
such as ZAP-70, CD38 and CD49d might take part in
strengthening the "proliferative" and/or "pro-survival"
interactions of CLL cells with microenvironment
[122,133,148,160] Such a "proliferative" status also
allows CLL cells to acquire additional/secondary genetic
changes, transforming them into a more aggressive
phe-notype [13]
Moreover, the expression of high levels of surface
mole-cules, such as CD38 and CD49d, may facilitate the
traf-ficking of CLL cells in the context of bone marrow and/or
lymph nodes where interactions with
microenvironmen-tal cells marked by "nurse-like" activities are easier to
occur [132,137-139,148] In this regard, it has been
hypothesized that the highest proliferation rate occurs
mainly/exclusively in the context of a tiny proportion of
tumor cells (i.e the so-called "tumor initiating cells" a.k.a
"cancer stem cells"), frequently clustered to form sort of
pseudofollicolar proliferation centers in lymph nodes and
bone marrow [139], but also present in peripheral blood
as "circulating cancer stem cells" with features of "side
population" in flow cytometry cytograms after fluorescent
vital dye staining [161]
Similar mechanism(s) might be hypothesized for M CLL
Also in this case, intrinsic and extrinsic factors may take
part in the neoplastic transformation but unlike UM CLL,
in M CLL the BCR might be selected by a sole antigen
(autoantigen or foreign antigen) or by a group of antigens
with very similar characteristics, often with evidence of a
geographic-biased distribution [92,105] This
"mono-reactivity" might determine a less aggressive pathology
[3,6,77] Of note, somatic hypermutation of IGV genes
can decrease autoreactivity levels [99] It is possible to
hypothesize that given the less aggressive clinical course,
in some cases CLL cells of a mutated clone may be anergic,
with an attenuated response to BCR engagement
[162-164] The low expression of CD38 and CD49d, usually
associated with a M IGHV gene status in CLL, fails to
pro-vide additional microenvironmental stimuli
The hypothesis of a "multistep" origin for CLL is in
keep-ing studies describkeep-ing the presence of B cells with CLL cell
features in about 3.5% of healthy people, allegedly
repre-senting a clonal amplification of a selected set of B
lym-phocytes [165,166]
Competing interests
The authors declare that they have no competing interests
Authors' contributions
MDB wrote the manuscript, FB, FF, AZ, RB, RM, SD, LL,
DGE, GG, GDP contributed to write the manuscript and
VG contributed to write and revised the manuscript
Acknowledgements
Supported in part by: Ministero della Salute (Ricerca Finalizzata I.R.C.C.S.,
"Alleanza Contro il Cancro", and Progetto Integrato Oncologia 2006) Rome, Italy; Programmi di Ricerca di Interesse Nazionale (P.R.I.N.) and Fondo per gli Investimenti per la Ricerca di Base (F.I.R.B.), M.U.R., Rome, Italy; Ricerca Scientifica Applicata, Regione Friuli Venezia Giulia, Trieste ("Linfonet"), Italy; Ricerca Sanitaria Finalizzata Regione Piemonte, Torino, Italy; Associazione Italiana contro le Leucemie, linfomi e mielomi (A.I.L.), Venezia Section, Pramaggiore Group, Italy; Novara-A.I.L Onlus, Novara, Italy; Siena-A.I.L Onlus, Siena, Italy; Associazione Italiana per la Ricerca sul Cancro (A.I.R.C.), Milan, Italy; Helmut Horten Foundation (Lugano, Swit-zerland); San Salvatore Foundation (Lugano, SwitSwit-zerland); Fondazione per
la Ricerca e la Cura sui Linfomi (Lugano, Switzerland); The Leukemia & Lymphoma Society (White Plains, NY).
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